Method and apparatus for pickling and for recovering spent acid solutions



ZJOSLME 2 Sheets-Sheet l C. B. FRANCIS ETAL AND APPARATUS FOR PICKLINGAND METHOD FOR RECOVERING SPENT ACID SOLUTIONS A A A- E A A A w `May 24,1955 Filed Aug. 13, 1948 May 24, 1955 C. B. FRANCIS ETAL METHOD ANDAPPARATUS FOR PICKLING AND FOR RECOVERING SPENT ACID SOLUTIONS FiledAug. 13, 194

2 Shaets-Sheet 2 wn FIGA- United States Patent O METFD ANR) APPARATUS FRPICKLlNG AND FR RECVERING SPENT ACD SOLUTIGNS Charles h. Francis andEdmund Lynch, Pittsburgh, Pa.,

assignors to United States Steel Corporation, a corporation of Newjersey Application August 13, 1948, Serial No. 44,108

14 Claims. (Cl. 134-13) This invention relates `to a method andapparatus for treating spent acid solutions and more particularly to thetreatment of spent solutions of sulphuric acid used in the continuouspickling of steel. In the continuous pickling of steel, the acidconcentration is ordinarily maintained above In order to maintain thisconcentration fresh acid is added to the last tank and the spentsolution is removed from the iirst tank. The spent acid solutionconsists of 18 to 20% by weight of ferrous sulphate, 5 to 6% of freesulphuric acid, other sulphates of metals which are alloyed with iron inthe steel, such as manganese, in about the proportions they occur in themetal, and the remainder of water. Such a solution has the specificgravity of about 1.2, B., and weighs approximately l0 pounds per gal. at4 C. This waste pickle liquor must be disposed of as a waste product,and in the past, most of the waste pickle liquor has been dumped intostreams, thus polluting them. More and :nere States are passing lawspreventing this dumping of waste pickle liquor and therefore variousprocesses have been proposed for disposing of the waste pickle liquor.In most instances, no profit is made from such processes and in manyinstances, the net cost is greater than the cost of dumping it intostreams. It has been proposed to cool the waste pickle liquor to removesome of the ferrous sulphate, but the methods proposed have not beensuccessful in practice.

It is an object of our invention to provide a method of pickling steelin which the waste pickle liquor is purified and returned to thepickling line.

Another object is to provide a method of recovering 5,.

These and other objects will be more apparent after I referring to thefollowing specification and attached drawings, in which:

Figure l is a schematic diagram of our invention as applied to acontinuous pickling line;

Figure 2 is a sectional View taken on the line lI-ll of Figure l;

Figure 3 is a sectional view taken on the line lll-III of Figure l; and

Figure 4 is a view, partly in section, of a modification.

Referring more particularly to the drawings, the reference numeral 2indicates an uncoiler from which a strip S to be pickled is uncoiledprior to the pickling thereof. Adjacent the uncoiler 2 are the bridlerolls 4 which feed the strip S to a welding machine 6 where the trailingend of one strip is fastened to the leading end of a following strip.From here the strip passes to a looper pit t! and thence through aseries of pickling tanks 10, 12, 14 and 16. Acid is washed from thestrip in the rinse tank 18 and the strip is then dried in the drier 20prior to recoiling on the coiler 22. A shear 24 is provided betweenthedrier 20 and coiler 22 to cut the strip into the ICC desired length. Theapparatus so far described is that commonly used for continuouslypickling strip. The acid flows from the tank 16 to tank 10 with freshacid being introduced into tank 16. The waste pickle liquor is removedfrom the tank 10 through a conduit 26 and is delivered into the upperend of a cooling tower 2.8, which may be of either the atmospheric orforced draft type. As shown, a fan 30 is provided at the upper endthereof to provide draft. Air inlets 32 and 34 are provided at the lowerend of the cooling tower Ztl which contains the usual bafes 36. Belowthe cooling tower 28 is a surge tank 38. The waste pickle liquor isintroduced into the bottom of the surge tank 3S and is withdrawntherefrom through the oating outlet of a tube made of rubber or othersuitable acid resistant material. A valve 42 is provided at the lowerend of tank 38. The acid leaving tank 10 is at a temperature within therange of 180 F. to 205 F. In tower 2S and tank 38 the waste pickleliquor is cooled to a temperature within 10 of the wet bulb temperatureof the air. In temperature zones this means that the temperature of thewaste pickle liquor will be between and 80 F. in the winter, and 70 F.in the spring and fall, and between and 100 F. in the summer. Sinceferrous sulphate begins to crystallize at 70 F. from spent acidsolutions containing as much as 20% of the salt, it is desirable duringcold seasons to regulate the cooling to prevent lowering the temperatureof the solution below 70 F. in the tank 3S.

Frein the tube lid the solution is pumped by means of a pump d4 to aV-shaped feed tank 46 whose top is open to the air conduit 34. A weirand liquid apron i3 is provided at the top of feed tank 46 and the wastepickle liquor flows over the weir into a distributing trough 50. Thistrough has aprons 52 on both sides thereof over which the waste pickleliquor ows in a thin sheet approximately 1/1@ to 1A; of an inch thick.The top edges of the trough are in a horizontal plane a few inches belowthe top edge of the weir 48. From the aprons SZ the spent acid dropsupon the top two boards 54 of a sloping double louver 56, which formsthe top section of a novel apparatus for cooling liquids. These boardsmay be made of any acid resistant material from 6 to 8 inches wide andmay vary in thickness up to 1 inch. The boards are supported one abovethe other a few inches apart with the top surface of the boards slopingto form an angle of 3 to 5 degrees from the horizontal so that the wastepickle liquor flows slowly across their surface. The number of boardsand the length and width of the louvers are determined by the volume ofspent acid to be treated per unit of time and are adjusted so that thedepth of the liquid flowing over the surface thereof does not eXceed 1/sinch. For example, to treat 50,000 gallons of spent acid per day, it isnecessary to provide a louver 20 feet long, 10 feet wide and about 2feet deep. In a louver with a frame of these dimensions, the number ofboards depends upon their width, at least 20 in each row being requiredwhen using boards 8 inches wide. When the waste pickle liquor is flowingslowly over the louver boards, a current of cool air passes thereover sothat the liquid is rapidly cooled due to the exchange of sensible heatfrom the liquid to the air. The air entering the louver is saturated`with moisture so that the total heat transferred from the liquiddepends largely upon the difference in temperature between the liquidand the air, the relative weight of the liquid and air, and the area ofthe exposed surfaces of the air and liquid. With the relative masses andtemperatures fixed, the rate of heat transfer is lconm trolled by theareas exposed. Thus, by regulating the flow of liquid and spreading itover a large surface, the rate of heat transfer is increased to amaximum and it is .a possible to cool the spent acid from 190 F. to 100F., from 100 F. to 60 F., or from 60 F. to 40 F. in 20 to 30 secondswhen the air is introduced at 0 F. In the particular example given, theliquid will llow from the end of the louver at a temperature no higherthan 70 F. into a vertical tank 58. A pair of butterfly valves 60 areprovided in the bottom of the tank 58 and are kept closed until thewaste pickle liquor rises to a level some distance below the bottom edgeof the louver, which distance in a commercial installation may beapproximately 4 or 5 feet. Air which has been compressed to 100 poundspressure and cooled to below minus 100 F. is then admitted into the tank58 through two rows of trumpet-shaped nozzles 62 and 64. By admittingthe air at this temperature and pressure through nozzles submerged inthe liquid, violent agitation and rapid cooling of the spent acid iseffected because the air expands forcing the liquid from the region ofthe nozzle and absorbing heat equivalent to that extracted aftercompression. Since the agitation of the liquid is violent, it causesintimate admixture of the air and waste pickle liquor so that the latteris cooled very rapidly at a ternperature which may be assumed to be 20-F. at a particular concentration. At this temperature a large part ofthe ferrous sulphate will separate from the solution in the form of verysmall crystals. If the weight of the air necessary to cool the wastepickle liquor from 70 F. to 20 F. in tank 58 is equal to the weight ofthe liquor passing through tank 38 it will lower the temperature ofspent acid on the louver 56 from about 110 F. to 75 F., the reductionbeing effected through an exchange of sensible heat suicient to raisethe temperature of the air from F. to 75 F. and the evaporation of about35 pounds of water per ton of air or liquid. Therefore, it is desirableto have the temperature of the solution entering the louverapproximately 100 F. so that it may leave at a temperature of about 70F. Under these conditions the amount of ferrous sulphate remaining inthe solution will vary from to about 8% according to the season of theyear. temperature of the liquid leaving the tank 58 can be obtained incold weather by using the fan 30. In hot weather the desired temperaturecontrol can best be obtained by increasing the amount of air compressed.

Below the cooler 58 is a pair of tanks 66 and 68. As soon as the liquidin the cooler 58 is cooled to the desired temperature, one of the valves60 is opened so as to permit the liquid to flow into one of the twotanks 66 or 68. When the first tank is full the valve 60 for the othertank is opened slightly until it is filled. Thereafter the valves overboth of the tanks may be left wide open and valves 70 at the bottom ofthe tank adjusted to permit the liquid to ow from the tanks at the samerate as it ows into the tanks. The tanks 66 and 68 are preferably madeof steel and lined with acid resistant material on the inside in thesame manner as the rest of the equipment, this being necessary toprevent attack by the acid. The tanks are insulated on the outside toavoid transfer of heat to the surrounding air, and are preferably largeenough to contain all the liquor treated during a period of 4 to 6hours. It is preferred to withdraw the solution from the two tanksalternately so as to allow time for crystal growth and to permit thecrystals to settle to the bottom, but not suflicient time for thecrystals to consolidate into a solid mass. The liquid and crystalswithdrawn from the bottom of the tanks are passed alternately into twocentrifugal type driers 72, which serve to separate the crystals fromthe cold mother liquor. The cold mother liquor is pumped from the drier72 by means of the pump 74 to the top of a heat exchanger 76. The liquidenters the heat exchanger at a temperature between F. and 30 F. andpasses over coils or through cells 78 made either of Mone] metal or ofsteel clad on the acid side with Monel metal. The compressed air used inthe cooling chamber 58 is delivered Better control of the nal to theheat exchanger 76 by means of a compressor 80 and passes through thecoils 78 and is discharged from the top of the heat exchanger 76 throughthe conduit 82. The heat exchanger is so designed that the temperatureof the air leaving the heat exchanger is below 100 F. and thetemperature of the liquid below 60 F. From the heat exchanger 76 theliquid is pumped by means of a pump 84 to the top of a second heatexchanger 86. The air to be cooled and compressed is delivered to thebottom of heat exchanger 86 by means of a first stage compressor 8S andthe cooled air passes from the top of the tank through the conduit 90 tothe second stage compressor 80. The heat exchanger 86 may be of varioustypes. In small scale operation good heat transfer can be had byspraying the liquid through sprays 92 into the chamber through which theair passes. Splash boards 94 are generally used with such sprays. Theliquid is preferably introduced at a pressure above pounds per squareinch and is withdrawn through the pipe 96 equipped with a pressureregulating Valve 98. To avoid injury to the second stage compressor 80,the heat exchanger 86 must be designed to prevent carryover of entrainedsolution. Not only is the air cooled in the heat exchanger 86, but thesolution is heated to a temperature between 70 F. and 90 F. Thissolution which contains from 6 to 8% of ferrous sulphate and 8 to 10% ofsulphuric acid and has approximately 80% of the original volume of thewaste pickle liquor in conduit 26 is conducted through the conduit 96 tothe last pickling tank 16. This tank 16 is preferably provided with atransverse rubber partition 100 about 15 feet from the end or the tankand the recovered acid is delivered to the tank at the exit end thereofas shown in Figure 1. Make-up acid and steam is introduced to the tank16 on the side of the partition 100 opposite the conduit 96. In this waythe drag-out loss consists almost entirely of recovered acid with thefresh make-up acid and water mixing with the recovered acid, thusdiluting the ferrous sulphate and increasing the efficiency of thesolution. In batch pickling, it is preferred to dip the sheets or othermaterial into a vat of recovered acid heated with steam to above F. andthen finish the pickling in fresh acid. However, this procedure is notalways advisable since the ferrous sulphate has an inhibiting etfectwhich can be taken advantage of by using the recovered acid as anintermediate or finishing bath.

In cooling the spent acid solutions to temperatures below 21 F., iceseparates and continues to separate as the temperature is loweredto 15F. This ice, which separates as small crystals suspended in thesolution, rises to the surface as a slush. In tanks 66 and 68, which areprovided with sloping4 tops, it is possible to remove this ice throughan outlet 102 at the highest point in the top of the tank. This slushcontains only a small amount of the acid and ferrous sulphate whichadheres to the ice crystals so that the removal of the slush results inconcentrating the acid and salt in the remaining solution, which inturn, causes more of the ferrous sulphate to crystallize. This ice maybe permitted to remain in the system as a temperature regulator,especially when operating 0n a minimum amount of compressed air. Ifdesired, the ice may be removed and used in the second heat exchanger 76to lower the temperature of the compressed air. When it is desired toreduce the quantity of the solution returned to the pickling baths, theice is kept out of the system entirely and used as a cooling mediumelsewhere.

It is also possible to recover manganese by the use of our invention.When initially starting the plant in operation, only ferrous sulphatecrystallizes out of the solution, but in time, manganese sulphateaccumulates in the solution until it begins to crystallize along withthe ferrous sulphate. When this condition is reached, a valve 104 inline 26 is closed and a quantity of hot spent acid is removed from thesystem through Conduit 106. `Flow of the solution in conduit 106 iscontrolled by valve 108. This solution is delivered to a manganesetreating plant 110 where it is treated with air and lime to precipitateiron, thus leaving the manganese in solution. Alternatively the wetcrystals may be dissolved in water, the ferrous sulphate oxidized toferrie sulphate, and the solution treated with a suitable base toprecipitate the iron and leave the manganese in solution.

1t will be understood that the described method may be applied in thetreatment of any spent acid solution produced in any way, not only forthe recovery of ferrous sulphate and sulphuric acid, but for therecovery of any salt, the solubility of which decreases with thetemper'- ature of the solution and any acid that remains in the solutionat the temperature at which the salt crystallizes. In place ofcompressed air, any other suitable gas may be used.

In place of using tanks 32 and 38 the spent pickle liquor may be cooledto between 100 F and 110 F. by ash operation by using the apparatusshown in Figure 4. in this embodiment of our invention the conduit 26 isconnected through a manifold 112 to two perforated pipes 114 and 116from which the spent acid drops upon horizontal splash boards 118 and120 located in a Vacuum chamber 122, From the ends of these splashboards the acid drops to a series of inclined cleated boards 124 whichspread the spent acid over a large surface and retard its flow throughthe vacuum chamber. The vacuum in the chamber 122 may be maintained inseveral Ways as by means of a water jet condenser 126. Water isintroduced into the water jet condenser 126 through a conduit 128 anddischarged therefrom into a trap 130.

In operation, the spent acid solution drawn into the vacuum chamber 122at a temperature between 180 F. and 205 F. will boil, thus filling thechamber with water vapor which passes from the chamber 122 through anoutlet 132 connected to the condenser 126 where it condenses, thusmaintaining a low pressure in chamber 122. As the only source of heat isthe solution itself and since heat is absorbed by the water invaporizing, the vaporized water cools the water in the liquid. Since thevapor pressure of water at 100 F. is about 55 millimeters of mercury andsince each pound evaporated at this temperature absorbs approximately1000 B. t. u., it is apparent that each pound of water evaporated willcool approximately pounds of water from 200 F. to 100 F. The vaporpressure of spent acid solutions is lowered considerably by the acidsand salts dissolved therein, the vapor pressure being lowered about l2millimeters for each 5% of acid and about l1 millimeters for each 15% offerrous sulphate in solution, or about 23 millimeters for both.Therefore, the solution gives off vapor rapidly in a vacuum at atemperature of 100 F. The operation requires some time since the vapormust be drawn off and condensed so that the actual drop of temperatureobtained also depends on the rate of flow through the apparatus. To holda vacuum in the chamber 122 the liquid must be pumped out as fast as itflows in. This removal may be accomplished by connecting a pump directlyto the bottom outlet 134 of the chamber 122. However, it is preferred toextend the outlet 134 to a trap 136 from which the liquid is withdrawnthrough a pipe 138 by means of pump 140, which delivers the cooled spentacid to the tower 46.

While several embodiments of our invention have been shown and describedit will be apparent that other adaptations and modifications may be madewithout departing from the scope of the following claims.

We claim:

l. The method of treating spent pickling solutions for the recovery offree acid and a salt thereof, which comprises exposing the solution toair at atmospheric temperature to cool the liquid to a temperature below110 F., expanding compressed air cooled to at least approximately 100 F.into a chamber containing the partially cooled Cit d liquid to cool theliquid to a temperature below 20 F., collecting the cold solution andpermitting it to stand until salt crystals of the desired size haveformed, and separating the crystals from the liquid.

2. The method of treating spent pickling solutions for the recovery offree acid and a salt thereof according to claim l, in which thecompressed air is cooled by exposing it to the liquid from which thesalt has been separated.

3. The method of continuously pickling steel which comprises removingthe spent acid solution from the entry end of the pickling line,exposing the solution to air at atmospheric temperature to cool theliquid to a temperature below 110 F., expanding compressed air cooled toat least approximately F. into a chamber containing the partially cooledliquid to cool the liquid to a temperature below 20 F., collecting thecold solution and permitting it to stand until salt crystals of thedesired size have formed, separating the crystals from the liquid,heating the liquid from which the salt has been separated by exposing itto the compressed air prior to its expansion into the chamber, returningthe reheated solution into the opposite end of the pickling line andpassing steel through the pickling line.

4. The method of continuously pickling steel according to claim 3 inwhich concentrated acid and hot water are added to the last named end ofthe pickling line to raise the acid concentration to the desired pointand to increase the volume of added fluids to that of the solutionremoved from the opposite end of the line.

5. in the method of continuously pickling steel, the improvement whichcomprises removing the spent acid solution from the entry end of thepickling line, expanding compressed air cooled to at least approximately100 into a chamber containing the liquid to cool the liquid to atemperature below 20 F., collecting the cold solution and permitting itto stand until salt crystals of the desired size have formed, separatingthe crystals from the liquid, heating the liquid from which the salt hasbeen separated by exposing it to the compressed air prior to itsexpansion into the chamber and returning the reheated solution into theopposite end of the pickling line.

6. In the method of continuously pickling steel according to claim 5,the additional step of adding concentrated acid and hot water to thelast named end of the pickling line to raise the acid concentration tothe desired point and to increase the volume of added fluids to that ofthe solution removed from the opposite end of the line.

7. Apparatus for cooling a liquid which comprises a tank for containingthe liquid, a chamber at the top of the tank extending upwardlytherefrom at an angle thereto, a sloping louver in said chamber overwhich the liquid flows into the tank, nozzles at the lower part of thetank for introducing compressed gas into said tank, a pair of tanks atthe lower end of said first tank in communication therewith, a valve foreach of said pair of tanks for controlling the flow of liquid thereto, asecond pair of tanks one above the other, means for introducing theliquid into the top of the first of said second pair of tanks, a gasconduit leading from the top of said louver to the bottom of said toptank, a conduit leading from the bottom of the top tank to the bottom ofthe lower of said second pair of tanks, and a liquid conduit leadingfrom the top of the lower tank to the top of the louver.

S. Apparatus for cooling a liquid according to claim 7 including a heatexchanger, a conduit leading from the bottom of each of the rst namedpair of tanks to the top of the heat exchanger, a compressor forintroducing the gas into the bottom of said heat exchanger, and a gasconduit leading from the top of said heat exchanger to the said nozzles.

9. Apparatus for cooling a liquid which comprises a tank for containingthe liquid, a chamber at the top of the tank extending upwardlytherefrom at an angle therer to, a plurality of solid boards arranged insaid chamber with their top surfaces arranged at a slight angle to thehorizontal and sloping in the same direction, the lower end of eachboard extending over the upper end of the board immediatelytherebeneath, the lower end of each board being spaced horizontally asubstantial distance from the board immediately thereabove, the liquidflowing over each board to the board immediately therebeneath and henceinto the tank, nozzles at the lower part of the tank for introducingcompressed gas into said tank, a pair of tanks at the lower end of saidiirst tank in communication therewith, a Valve for each of said pair oftanks for controlling the ilow of liquid thereto, a vacuum chamber, aconduit for introducing the heated liquid into said Vacuum chamber, anda liquid conduit leading from said vacuum chamber to the top of theboards.

10. Apparatus for cooling a liquid according to claim 9 including a heatexchanger, a conduit leading from the bottom of each of the iirst namedpair of tanks to the top of the heat exchanger, a compressor forintroducing the gas into the bottom of said heat exchanger, and a gasconduit leading from the top of said heat exchanger to the said nozzles.

11. Apparatus for cooling a liquid which comprises a tank for containingthe liquid, a chamber at the top of the tank extending upwardlytherefrom at an angle thereto, a plurality of solid boards arranged insaid chamber with their top surfaces arranged at a slight angle to thehorizontal and sloping in the sante direction, the lower end of eachboard extending over the upper end of the board immediatelytherebeneath, the lower end of each board being spaced horizontally asubstantial distance from the board immediately thereabove, the liquidiiowing over each board to the board immediately therebeneath and henceinto the tank, nozzles at the lower part of the tank for introducingcompressed gas into said tank, a pair of tanks at the lower end of saidrst tank in communication therewith, a valve for each of said pair oftanks for controlling the flow of liquid thereto, a heat exchanger, aconduit leading from the bottom of each of the pair of tanks to the topof the heat exchanger, a compressor for introducing the gas into thebottom of said heat exchanger, and a gas conduit leading from the top ofsaid heat exchanger to the said nozzles.

12. Continuous strip pickling apparatus comprising a series of picklingtanks through which the strip passes, a louver, a conduit leading fromthe pickling tank at the strip entry end to the top of the louver ,forintroducing the waste pickle liquor thereto, a tank beneath the louvercommunicating therewith, nozzles at the lower part of the tank forintroducing compressed gas into said tank, a pair of tanks at the lowerend of said iirst tank in communication therewith, a valve for each ofsaid pair of tanks for controlling the flow of liquid thereto, a heatexchanger, a conduit leading from the bottom of each of the pair oftanks to the top of the heat exchangena compressor for introducing gasto be cornpressed into the bottom of said heat exchanger, a gas conduitleading from the top of said heat exchanger to the said nozzles, and aconduit leading from the bottom of the heat exchanger to the picklingtank at the strip exit end.

13. Continuous strip pickling apparatus according to claim 12 includingmeans in the rst said conduit for cooling the liquor to a temperaturebelow F.

14, Continuous strip pickling apparatus comprising a series of picklingtanks through which the strip passes, means for cooling the waste pickleliquor to a temperature below 110 F., a conduit leading from the tank atthe strip entry end to the said cooling means, a tank for the liquor, asecond conduit leading from the cooling means to said last-named tank,nozzles at the lower part of the last-named tank for introducingcompressed gas into said last-named tank, a heat exchanger, a thirdconduit leading from the bottom of said last-named tank to the heatexchanger, and a conduit leading from the bottom of the heat exchangerto the pickling tank at the strip exit end for delivering the liquorthereto.

References Cited in the file of this patent UNITED STATES PATENTS310,025 Brewer Dec. 30, 1884 489,147 Golding Ian. 3, 1893 1,146,071Hoiman July 13, 1915 1,172,133 Grevemberg Feb. 15, 1916 1,207,133 ByardDec. 5, 1916 1,359,911 Oman Nov. 23, 1920 1,392,780 Marsh Oct. 4, 19211,738,543 Travers Dec. 10, 1929 1,866,443 Zumbo July 5, 1932 1,962,295Bruins June 12, 1934 2,155,854 Barnes Apr. 25, 1939 2,316,744 Mellett etal. Apr. 13, 1943 2,321,885 Allen June 15, 1943 FOREIGN PATENTS 839,695France Jan. 7, 1939 OTHER REFERENCES Thermodynamics, by Emswiler, Secondedition, 1927, published by McGraw-Hill, pp. 157, 158 and 159 reliedion.

Hackh, Chemical Dictionary, Third edition, 1944, page 16S. BlakistonCo., Philadelphia.

3. THE METHOD OF CONTINUOUSLY PICKLING STEEL WHICH COMPRISES REMOVINGTHE SPENT ACID SOLUTION FROM THE ENTRY END OF THE PICKLING LINE,EXPOSING THE SOLUTION OF AIR AT ATMOSPHERIC TEMPERATURE TO COOL THELIQUID TO A TEMPERATURE BELOW 110* F., EXPANDING COMPRESSED AIR COOLEDTO A LEAST APPROXIMATELY-100* F. INTO A CHAMBER CONTAINING THE PARTIALLYCOOLED LIQUID TO COOL THE LIQUID TO A TEMPERATURE BELOW 20* F.,COLLECTING THE COLD SOLUTION AND PERMITTING IT TO STAND UNTIL SALTCRYSTALS OF THE DESIRED SIZE HAVE FORMED, SEPARATING THE CRYSTALS FROMTHE DESIRED SIZE HAVE FORMED, SEPARATING THE CRYSTALS FROM THE LIQUIDS,RATED BY EXPOSING IT TO THE COMPRESSED AIR PRIOR TO ITS EXPANSION INTOTHE CHAMBER, RETURNING THE REHEATED SOLUTION INTO THE OPPOSITE END OFTHE PICKLING LINE AND PASSING STEEL THROUGH THE PICKLING LINE. 12.CONTINUOUS STRIP PICKLING APPARATUS COMPRISING A SERIES OF PICKLINGTANKS THROUGH WHICH THE STRIP PASSES, A LOUVER, A CONDUIT LEADING FROMTHE PICKLING TANK AT THE STRIP ENTRY END TO THE TOP OF THE LOUVER FORINTRODUCING THE WASTE PICKLE LIQUOR THERETO, A TANK BENEATH THE LOWERCOMMUNICATING THEREWITH, NOZZLES AT THE LOWER PART OF THE TANK FORINTRODUCING COMPRESSED GAS INTO SAID TANK, A PAIR OF TANKS AT THE LOWEREND OF SAID FIRST TANK IN COMMUNICATION THEREWITH, A VALVE FOR EACH OFSAID PAIR OF TANKS FOR CONTROLLING THE FLOW OFLIQUID THERETO, A HEATEXCHANGER, A CONDUIT LEADING FROM THE BOTTOM OF EACH OF THE PAIR OFTANKS TO THE TOP OF THE HEAT EXCHANGER, A COMPRESSOR FOR INTRODUCING GASTO BE COMPRESSED INTO THE BOTTOM OF SAID HEAT EXCHANGER, A GAS CONDUITLEADING FROM THE TOP OF SAID HEAT EXCHANGER TO THE SAID NOZZLES, AND ACONDUIT LEADING FROM THE BOTTOM OF THE HEAT EXCHANGER TO THE PICKLINGTANK AT THE STRIP EXIT END.